This is a "Cartridge SA Observation .303 inch O Mark I"
It had a 174gr. bullet with a small hole in the tip which was filled with fusible metal. The only headstamps I am aware of are "R^L 29, “R^L 37 OI” and “R^L 39 OI”. There are 2 variations of the 1939 headstamp with a wider and closer spacing between the “R^L” and “39”. The cartridge was approved for service in October 1934.